'Alien: Romulus' has the same problem as 'Prometheus' and 'Alien: Covenant' — and it's bad news for the franchise 

 The magical black goo introduced in "Prometheus" is holding the Xenomorph back. 

https://cdn.mos.cms.futurecdn.net/2h6NjaStsH3p4uGMVdKbc5.jpg

https://www.space.com/alien-romulus-has-the-same-problem-as-prometheus-and-alien-covenant